This invention relates to pipeline systems particularly designed for transporting very low temperature or cryogenic fluids, especially natural gas, usually essentially in liquid form and termed herein LNG, and is particularly concerned with the provision of a pipeline system of the above type, including means for reducing thermal stress and heat leak in the system during passage of LNG therethrough.
The temperature of liquefied natural gas (LNG) at atmospheric pressure is about -260.degree. F. Only a few materials, such as aluminum or high nickel austenitic stainless steels, 9% nickel steel, or "Cryonic 5" maintain their toughness at this temperature. If restrained at both ends and cooled to -260.degree. F., all of these materials would develop thermal stress as result of contraction, higher than allowable for the design of cryogenic pipe.
Cryogenic piping is normally designed within allowable stress levels by one of two methods. The more usual method is to obtain piping flexibility by change of direction of the piping, such as by means of right angle bends, U-bends or Z-bends. This method uses an increased length of pipe in offset, to reduce the thermal stresses and loads due to temperature cycling. Essentially, the thermal strain is taken up as a bending deflection, with the pipe absorbing the deflection made sufficiently long to keep the bending stresses within allowable limits. The U-bends, or expansion loops, in steam pipes is an example of this method of designing for an allowable amount of thermal strain. The second method is to employ expansion joints which use flexible bellows to compensate for the axial thermal strain. A third method, used only in very special cases, is the use of low expansion coefficient material in the piping. Invar is a high nickel content iron-nickel alloy with a low coefficient of thermal contraction. It has been used for cryogenic piping in cases where offset lengths of piping or bellows were not allowable. It is, however, too costly for most situations. These methods are not acceptable for buried or underwater pipelines.
Moreover, such prior art means for temperature compensation of the LNG line not only are expensive and cumbersome, but also present a potential leakage problem. The situation is compounded where, for example, temperature compensation is required for very long lengths of the order of thousands of feet of LNG pipeline.
To overcome the above disadvantages there has been developed in the prior art the dual concentric prestressed pipeline concept. According to this concept, coaxial inner and outer pipes are provided which are connected together at their ends, with one of the pipes, e.g. the inner pipe, being prestressed and placed under axial compression while the other pipe, e.g. the outer pipe, is placed under axial tension. Illustrative of such prior art are U.S. Pat. No. 3,530,680 to Gardner and U.S. Pat. No. 3,693,665 to Veerling.
An improvement of such dual concentric prestressed pipeline concept is described in U.S. Pat. No. 3,865,145 to McKay et al, and assigned to the same assignee as the present application. The latter patent discloses a pipeline system for transporting LNG comprising an inner pipe and an outer pipe, the outer pipe being disposed concentrically about the inner pipe and a plurality of stress cones in the annular space between the inner and outer pipes, and securing the inner pipe to the outer pipe, and arranged to transfer a compressive load from one pipe to a tension load on the other pipe. An insulator member formed of a material such as Teflon is added at the connection of the stress cones to either the outer or inner pipe. However, it has been found in this design that the stresses in the inner or LNG carrier pipe, and heat leak along the cones are higher than is desirable.
It is an object of the present invention to provide an improved cryogenic pipeline system of the general type described in the above McKay et al U.S. Pat. No. 3,865,145, having reduced stresses, particularly in the inner carrier pipe, and having reduced heat leak between the outer and inner pipes, particularly along the connecting members or cones. Another object is the provision of a pipeline system as noted above, wherein working stresses are within acceptable limits, and are fully predictable, allowing for adequate safety factors. A still further object is to provide a pipeline system of the foregoing type, which is suitable for installation under conditions which essentially eliminate accessibility for maintenance once the installation has been made, that is for use under water or buried, or both. Yet another object is the provision of a pipeline system of the aforementioned type, having suitable insulation to minimize heat absorption and frost heaving of the surrounding fill. Yet another object is to provide a pipeline system of the above type of relatively simple design and fabricated from suitable metals which are commercially available, and which permits facile fabrication of the pipe sections at a factory and only requires assembly by welding of the pipe sections in the field or on site. A still further object is the provision of a pipeline system of the type noted above which is designed to essentially eliminate leaks.